https://github.com/Simplistix/testfixtures/issues/115#issuecomment-494677456 Right, what happens if you use the latest mock backport, rather than one that's 3 years old? ;-) https://pypi.org/project/mock/3.0.5/ Блокирует сборку новой версии python-module-testfixtures
Надо использовать unittest.mock из Python 3.3
